The present disclosure relates to a back side illumination image sensor.
Image sensors are semiconductor devices that convert optical images into electric signals and are generally classified as a CCD (Charge Coupled Device) image sensor or a CMOS image sensor (CIS).
A photodiode of a related art CIS is disposed on a substrate by an ion implantation process. As the size of the photodiode gradually decreases to increase the number of pixels without increasing the chip size, there is a growing tendency that image quality is reduced due to reduction of the area of an illumination part.
Further, since the stack height is not reduced as much as the reduction of the area of the illumination part, there is tendency that the number of photons entering the illumination part is reduced by diffraction of light, which is called “airy disk”.
In order to address the above problem, there is provided a backside illumination image sensor, which receives light through a wafer backside to minimize a step difference at an upper portion of a light receiving section and avoid light interference caused by metal routing.
FIG. 1 is a cross-sectional diagram illustrating a back side illumination image sensor according to the related art.
In the back side illumination image sensor according to the related art, an illumination device and interconnection are formed on the front side of a substrate and then a back grinding for removing the rear side of the substrate to a predetermined thickness is performed. This back grinding process of the rear side of the substrate is for fitting the gap between an external module and an optical lens to an appropriate thickness.
However, in the back side illumination sensor according to the related art, an SOI (Silicon On Insulator) wafer is used as a donor wafer where an illumination device and a circuit part are disposed, and then the SOI wafer is bonded to a handle wafer. Thereafter, a back side thinning process is applied to the donor wafer.
The backside thinning process that is applied to the donor wafer according to the related art is as follows.
First, the backside grinding process is applied to the donor wafer such that several tens of μm remains on the upper portion of a BOX (Buried Oxide) layer of the SOI wafer. Thereafter, the backside thinning process is completed by performing etch-back.
However, according to the related art, since an expensive SOI wafer is used for the donor wafer, the cost for the manufacturing process increases.
Further, according to the related art, as shown in FIG. 1, wafer edge thinning is generated by the backside grinding process of the donor wafer. Accordingly, a failure may occur in the chip at the wafer edge in the etch-back process performed after the backside grinding process, resulting in a problem that economical efficiency is considerably deteriorated.
Further, according to the related art, the wafer center is also exposed to plasma damage in the etch-back process of several tens of μm, resulting in a problem that sensor performance may be deteriorated.
Further, according to the related art, since a margin for opening a pad may be insufficient in a pad-opening process, there is a problem that a metal for the pad contact may be perforated or not opened.
Meanwhile, according to the related art, the photodiode may be deposited by using amorphous silicon. Otherwise, after a readout circuitry is formed on a silicon substrate, and the photodiode is formed on another wafer, the photodiode is formed over the readout circuitry through a wafer-to-wafer bonding scheme to form an image sensor (hereinafter, referred to as “3D image sensor”). In this case, the photodiode and the readout circuit are connected by a metal line.
However, according to the 3D-image sensor of the related art, when bonding the wafer with the readout circuit to the wafer with the photodiode, it can be difficult to completely electrically connect the readout circuit with the photodiode due to a problem relating to bonding. For example, according to the related art, a metal line is formed on the readout circuit and wafer-to-wafer bonding is performed such that the metal line contacts with the photodiode. However, not only does the metal line not completely contact with the photodiode, but it is difficult to implement an ohmic contact between the metal line and the photodiode. Further, according to the related art, a short may be generated in the metal line that is electrically connected with the photodiode. Accordingly, researches for preventing the short have been conducted, but the process becomes complicated.